Abstract:

The resistance of cold-formed thin-walled cassettes against local transverse forces, i.e. the web crippling capacity, was investigated. The web crippling of cassettes, or liner trays as they are also called, was studied both experimentally and numerically using finite element modelling. Both unreinforced (flat) cassette webs and webs with longitudinal stiffeners situated on only one side of the web mid-line were studied.

The calculation of the web crippling capacity of this type of stiffened webs is not included in current design codes. However, if cassettes are designed as continuous over two or more spans, the resistance against local transverse forces has to be verified. It should be noted that the original purpose of the stiffener in cassette webs is to increase the bending moment capacity by increasing the effective area of the compressed part of the web when cassettes are designed as single-span structures, in which case web crippling does not usually become critical.

A total of 52 structural tests were carried out on specially manufactured single cassette web sections and built-up cassette structures. Both interior two-flange and interior one-flange loading were considered. The calculation rules for unreinforced webs given in current design codes were found to be relatively conservative in comparison to the test results.

Finite element models were developed and validated on the basis of the test results and very good agreement was achieved. These models were used as a starting point for a parametric study of the influence of different cross-sectional parameters on the resistance against local transverse forces of longitudinally stiffened webs. Also the influence of the load bearing length was included in the study. Recommendations concerning the design of the cassette web section are given based on the results.

It was shown that the use of a longitudinal stiffener of the studied type in fact reduces the web crippling capacity by at least 10 % in comparison to a similar cassette with an unreinforced web. Depending on the stiffener's geometry, the reduction can be considerably larger. A reduction factor equal to 0.7 - 0.9 should be used for the type of longitudinally stiffened webs considered in this study in connection with the design code formulae for the resistance of unreinforced webs against local transverse forces, depending on the cross-section geometry.